Research on dual-output port on-board charging system based on CLLC resonant converter

This paper proposes a dual-output port on-board charging system based on a CLLC resonant converter, which can realize the function of simultaneously charging high-voltage power batteries and low-voltage batteries. The system topology is composed of a front-stage PFC and a latter-stage CLLC resonant converter. The system uses a three-port transformer to couple the resonant converter, the high-voltage charging circuit, and the low-voltage charging circuit. The system also realizes soft switching through resonant components. According to the design requirements of the charging system, the control strategy of the dual-output ports is determined by analyzing the working mode and gain characteristics of the system and the dual closed-loop control structure is designed to realize the functions of power factor correction and wide-range voltage regulation. Simulation and experimental results verify the correctness of theoretical analysis and structural design.


Introduction
With the increasing prominence of global environmental issues and the increasing popularity of electric vehicles, many auto companies focus on the design and development of electric vehicles [1,2]. At present, vehicle-mounted power batteries and their charging technology have attracted much attention. Vehicle-mounted power batteries determine the performance indicators of electric vehicles, and charging technology directly affects the performance of power batteries [3,4]. Fig 1 shows the typical electric structure of pure electric vehicle (PEV), which mainly includes the on-board battery charger (OBC), motor drive controller and battery manage system (BMS), etc. As the core of energy conversion of electric vehicles, the OBC provides power for high-voltage power battery and low-voltage battery, which are detected and managed by BMS for charge quantity, while the motor driving part is controlled by motor driving controller [5].
As the key technology of pure electric vehicle drive system, the technical level of electric drive system directly determines the power and economy of PEV. At present, according to the layout of the drive system, PEV can be divided into centralized drive layout and distributed drive layout. The centralized drive system mainly uses the drive motor and reducer to replace PLOS ONE PLOS ONE | https://doi.org/10.1371/journal.pone.0279558 February 24, 2023 1 / 29 a1111111111 a1111111111 a1111111111 a1111111111 a1111111111 the powertrain system of the traditional fuel vehicle to realize the electrification of the drive system, while the distributed drive is to install the drive motor directly on the wheel to drive the vehicle forward. Its structure and characteristics are shown in Table 1.
As a device that is directly connected to the power grid, the on-board charging system mostly adopts a two-stage structure. The front-stage power factor correction (PFC) solves the interference problem of the charging system to the grid, suppresses the harmonics of input current, and provides a stable voltage for the later-stage DC bus. In order to adapt to the change of load during the charging process, the latter-stage DC-DC converter converts the  constant voltage output by the front stage into a wide range of output voltage, and reduce the ripple of the output voltage for the front-stage PFC [6]. The front-stage PFC is generally a nonisolated converter, such as a Buck converter, a Boost converter, a derived bridgeless PFC, etc. [7]. The latter-stage DC-DC converter topology is divided into isolated converters and nonisolated converters. Isolated converters have received widespread attention because of their electrical isolation characteristics [8].
The high-voltage power battery charging circuit and the low-voltage battery charging circuit are important components of electric vehicles, but both sides are often operated separately, which have disadvantages such as large space occupation, high cost, and low efficiency. Domestic and foreign scholars have carried out research on the aforementioned problems. Mallik A proposed a Buck-Boost-based non-isolated dual-output port converter, which uses pulse width modulation (PWM) to achieve wide output voltage gain. However, it uses a high number of components and there is a problem in that the power switches are hard-switching, which causes a large switching loss [9]. Chen G proposed a non-isolated dual output port converter based on Cuk, which only uses two power switches, two diodes, three inductors, and three capacitors. The number of components used is reduced, but the problem of large switching loss in the power switches is introduced [10]. Prabhakaran P proposed a dual-input, dualoutput non-isolated converter, which uses only one inductance element, which greatly reduces the volume of the converter, but both the current stress and switching loss are greater [11]. In order to reduce the switching loss of the power switches, Wu H proposed isolated full-bridge three-port converters, which uses PWM control to achieve zero voltage switching (ZVS) of the power switches on the primary side of the transformer. The switching loss of the power switches is reduced, but the converter gain range is lower [12]. Dao N proposed an isolated three-port converter based on series resonance and dual active bridge (DAB) structure, using a control strategy that combines phase-shift and frequency modulations to achieve wide output voltage gain, but its control strategy is more complicated and uses more components [13]. Oluwasogo E S adopted a self-current balancing structure of the dual-transformer-based triple-port active bridge (DT-DAB) converter. In comparison to the traditional DAB, it reduces the number of components, and adds an additional degree of freedom (DOF) on the primary side of H bridge, which expands the converter operation range and realizes ZVS of all power switches. However, increasing the amount of control increases the difficulty of system control and makes the control strategy more complicated [14]. Zhou K proposed a dual output port converter, which integrates high and low voltage circuits in an LLC resonant circuit and achieves a wide range of voltage output through dual closed-loop PI control. However, the low-voltage side of the circuit is an ordinary full-bridge circuit and does not have soft switching characteristics [15]. This paper proposes a dual-output port on-board charging system based on a CLLC resonant converter. The front-stage adopts a dual-diode bridgeless PFC, which improves the power factor while reducing common mode interference and outputs a stable DC bus voltage on latter-stage. The latter-stage integrates the high-voltage charging circuit with the low-voltage charging, and is coupled through a three-port transformer to meet the requirements of high efficiency and high power density of the on-board charging system [16]. In order to reduce switching loss and improve the efficiency of charging and discharging, the main topology adopts the CLLC resonant converter. Compared with the traditional LLC resonant converter, the CLLC resonant converter adds resonant components on the secondary side to ensure that all power switches can achieve ZVS during the battery charging process and battery discharge process, and further reduce the switching loss [17]. The control strategy adopts double closed-loop PI control to realize the power factor correction function of the front-stage circuit and the function of high-efficiency and wide-range output voltage of the latter-stage [18].
2 Structure of the on-board charging system Fig 2 shows the proposed topological structure of the dual-output on-board charging system, which includes a transformer primary-side circuit connected to a PFC circuit, a high-voltage charging circuit connected to a high-voltage power battery, and a low-voltage charging circuit connected to a low-voltage battery. The primary side circuit, high-voltage charging circuit, and low-voltage charging circuit are coupled through a transformer.

Power factor correction circuit
The power factor correction circuit primarily consists SiC-MOSFET S 1 and S 2 , Schottky diodes D 1~D4 , DC bus capacitor C b , and two energy storage inductors L 1 and L 2 . The front-stage circuit adopts a double-diode bridgeless PFC, and the input AC voltage source (V ac ) are connected to D 1 and D 2 . During operation of the circuit, there are only two semiconductor devices in the current flow path, so the switching loss is small, the efficiency is high. D 1 and D 2 connect the ground on the DC bus to the grid side, which can effectively suppress the generation of common mode interference, reduce the total harmonic distortion (THD) on the input side of the rectifier network, and provide a stable DC bus voltage for the latter-stage.

Primary-side circuit
The primary-side circuit primarily consists of MOSFET Q 1~Q4 , resonant inductor L r1 , resonant capacitor C r1 , and transformer primary winding W 1 , Q 1~Q4 form a full-bridge circuit, L r1 and C r1 form a resonant circuit. The full-bridge circuit and the resonance circuit are resonant for converting the front-stage circuit of output DC bus voltage into high frequency voltage pulses and pass the high-frequency voltage pulse to the high voltage charging circuit and the low-voltage charging circuit through the winding W 1 .

High-voltage charging circuit
The high-voltage charging circuit primarily consists of MOSFET Q 5~Q8 , resonant inductor L r2 , resonant capacitor C r2 , sustaining capacitor C 0 , and transformer secondary winding W 2 , Q 5~Q8 form a full-bridge circuit, L r2 and C r2 from a resonant circuit, and the output port of the circuit is connected to the high-voltage power battery. When the high-voltage power battery is in the charging state, the full-bridge circuit and the resonant circuit are responsible for converting the high-frequency voltage pulse output by the winding W 2 into a DC voltage, and by maintaining capacitor C 0 to ensure that the output voltage is stable. When in the discharging state, the full-bridge circuit and resonance circuit convert the output voltage of the highvoltage power battery into high-frequency voltage pulses and pass the high-frequency voltage pulses to the low-voltage charging circuit through the winding W 2 .

Low-voltage charging circuit
The low-voltage charging circuit primarily consists of MOSFET Q 9 and Q 10 , filter inductor L f , filter capacitor C f , and transformer secondary winding W 3 . Q 9 and Q 10 form a full-wave rectifier circuit, L f and C f form a filter circuit. The output of the circuit is connected to the low voltage battery. The full-wave rectifier circuit converts the high-frequency voltage pulse output by winding W 3 into a DC voltage, and filters high-order harmonics through a filter circuit to ensure stable output voltage.

Operating mode
The system has two working modes: parked charging mode and driving charging mode. Fig 3 shows the parked charging mode. When the BMS detects that the power of the high voltage power battery is insufficient, the switching frequency of Q 1~Q4 is stable near the resonant frequency and Q 5~Q8 are in the synchronous rectification working state. At this time, the energy is transferred from the primary-side circuit to the high-voltage charging circuit through T 1 to charge the high-voltage power battery. When the BMS detects that the low-voltage battery is low, the switching frequency of Q 1~Q4 is stable near the resonant frequency, where Q 9 and Q 10 are in the synchronous rectification state. At this time, the energy is transferred from the primary-side circuit to the low-voltage charging circuit through T 1 to charge the high-voltage power battery. The external control circuit adjusts the frequency of Q 1~Q4 to realize the regulation of the output voltage of the high-voltage charging circuit and the low-voltage charging circuit.
When the system works in the driving charging mode, Q 1~Q4 are in the off state. At this time, the high-voltage charging circuit and the low-voltage charging circuit form a series resonant circuit, as shown in Fig 4. When the BMS detects that the low-voltage battery has insufficient power, the switching frequency of Q 5~Q8 is stable near the resonant frequency, where Q 9 and Q 10 are in the synchronous rectification state. At this time, energy is transferred from the high voltage charging circuit to the low-voltage charging circuit through T 1 to charge the lowvoltage battery. The external control circuit uses pulse frequency modulation (PFM) to control the frequency of Q 5~Q8 to adjust the output voltage of the low-voltage charging circuit. R eq is the equivalent resistance of the high-voltage side converted to the primary side, and its equation is as follows:

The voltage gain characteristics of the system
where, n is the transformer turn ratio, U out , I out and P out are the output voltage, current and power of the high-voltage side, respectively.

PLOS ONE
Make the CLLC converter resonant component parameters symmetrical, as shown below: where, L r1 and C r1 are the primary-side resonant inductor and resonant capacitor, L r2 and C r2 are the secondary side resonant inductors and resonant capacitors.
The DC voltage gain characteristics of the CLLC resonant converter are obtained as follows: Mðf n ; Q; kÞ ¼ 1 ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffiffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi where, M is voltage gain, f n is the normalized frequency, Q is the quality factor, and k is the inductance coefficient.
Here, f r1 is the resonant frequency of the converter, its value is 100 kHz, and f s is the actual switching frequency.
Here, L m is the magnetizing inductor.
From Eq (3), the converter voltage gain expression contains three parameters, and direct analysis will lead to redundant parameter design and complicated analysis process. This paper proposes a simplified analysis method, through to formulate a parameter values for the analysis of the other two parameters and the relationship between the voltage gain,. Since k is relatively easy to determine compared with Q and f n , this paper firstly uses the provisional k value to analyze the change relationship between M and f n and Q, then the provisional Q value to analyze the change relationship between M and f and k, and finally determines the appropriate k value and Q value by combining the analysis of the two. The specific parameter design process is shown in Fig 6. The usual empirical value of inductance is 1 to 7, and the larger the value of k, the smaller the resonant current value and the smaller the loss. However, a larger value of k will widen the frequency range of the converter and reduce the service life of magnetic components. The empirical value of inductance is usually chosen as 5 in engineering, so k = 5 is drawn up in this paper, and the relationship between converter voltage gain M, f n and Q can be obtained, as shown in   Define the normalized first resonant frequency and second resonant frequency as f n1 and f n2 as follows: When k = 5, the influence of two resonant frequencies and quality factor Q on the gain curve is shown in Fig 8.  Fig 8 shows that the larger the value of Q, the smaller the gain, and the lower the frequency sensitivity. The smaller the value of Q, the greater the gain and the higher the frequency sensitivity, but it is unstable. In order to realize the accuracy of closed-loop voltage regulation control, it is necessary to ensure that the voltage gain decreases monotonously as the frequency increases during the working process of the converter. At the same time, in order to meet the soft switching conditions, the circuit should be in an inductive working state. The key is that the input impedance is inductive. For the CLLC resonant converter, its input impedance expression is as follows: Let the imaginary part of Z in be 0. Then the maximum quality factor Q max of the circuit working in the inductive range is as follows: Q max ¼ f n ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi The minimum value is as follows: When k = 5, the minimum value of Q is 0.437. In order to weigh the influence of the inductance coefficient on the voltage gain, when Q = 0.3, the relationship curve between the converter voltage gain M, f n and Q can be obtained, as shown in Fig 9.  Fig 9 show that as the value of k decrease, its maximum gain range increases and the frequency sensitivity also increases, but a smaller k will make gain stability worse. A larger value of k will result in insufficient gain or a non-monotonous decreasing trend of voltage gain with increasing frequency, which increases the difficulty of closed-loop control. Table 2 shows the design parameters of the dual-output port on-board charging system.

AC input inductor
Since the switching frequency is much higher than the input voltage frequency in the same switching cycle, it can be assumed that the input voltage does not change, and the circuit can be regarded as a Boost converter. When the power switches are turned on, the calculation formula of the inductor voltage is as follows:  Here, U in is the input voltage, D is the duty cycle, T sw is the switching period, ΔI is the inductor current ripple, and the ripple coefficient should not be greater than 20%, which can be obtained as follows: Here, I p is the peak value of the input current, P is the rated power, U bus is the output DC bus voltage, η is the working efficiency of the PFC, generally not lower than 0.95 under full load conditions, and the inductor L is obtained by incorporating the system design parameters as follows:

Transformer ratio
According to the design requirements of the system parameters, the converter voltage gain must be the largest when the DC bus voltage is 400V and the high voltage output is 400V. The ratio n 1 of the transformer primary winding W 2 to the high voltage side winding as follows: where, U bus_max is the maximum value of DC bus voltage and U H_max is the maximum value of high-voltage output.
In order to ensure that the low-voltage electrical equipment in the car can operate normally when the high-voltage power battery is low, the transformation ratio n 2 of the high voltage side winding and the low voltage side winding W 3 is as follows: where, U H_min is the minimum output voltage on the high-voltage side and U L_max is the maximum output voltage on the low-voltage side.

Maximum gain and minimum gain
When the system charges the vehicle power battery, its maximum gain M H_max and minimum gain M H_min are as follows: where, U H_max is the maximum output voltage on the high voltage side, U bus_min is the minimum input voltage on the DC bus side, and U of is the switch voltage drop (U of = 1V).
When the system is charging the low-voltage battery on the vehicle, its maximum gain M L_max and minimum gain M L_min are as follows: where, U L_min is the minimum output voltage on the low-voltage side and U of is the power switch voltage drop (U of = 1V).

Resonant inductor, resonant capacitor, and magnetizing inductor
It can be deduced from Figs 8 and 9 that the inductance coefficient k satisfies the system's requirements of maximum gain, minimum gain, and the monotonicity of gain, At this time, it is necessary to ensure that the circuit operates in ZVS inductive zones 1 and 2 and the quality factor Q is not greater than Q max (0.437). However, the larger the value of Q, the lower the frequency sensitivity and the better the stability. Therefore, the value of Q should be as large as possible and the value of Q should be 0.42 according to the design. From Eqs (5) and (6), the calculation formulas for resonant inductor, capacitance, and magnetizing inductor are as follows: In order to meet the ZVS of the power switches, the dead time is set to 200ns. Fig 10 shows the control strategy of the dual output port vehicle charging system. The control part mainly includes a dual-diode bridgeless PFC circuit controller, a parked charging mode controller and a driving charging mode controller. The dual-diode bridgeless PFC controller adopts double closed-loop control based on the average current method. Its voltage outer loop is the difference between the output bus voltage (U bus ) of the previous stage and the reference voltage (U bus_ref ). After PI adjustment, it is multiplied with the modulus of the input voltage (|U ac |) and finally the current inner loop reference voltage is obtained. The current inner loop is the difference between the reference current and the modulus of input current (|I ac |). The duty cycle signal is obtained through PI adjustment. The duty cycle of S 1 and S 2 is controlled by PWM to realize the power factor correction function.

System control strategy
Both the parked and the driving charging mode controllers adopt voltage-current double closed-loop control. The output voltage and current are introduced as the control variables of the voltage outer loop and the current inner loop for PI adjustment and the power switches frequency is controlled by PFM. Therefore, the adjustment function of the output voltage is realized. Its charging mode adopts the strategy of constant-current constant-voltage (CC/CV) and uses the limiting secondary link to limit the output value of the voltage outer loop. That is, the voltage outer loop output value is greater than I H_ref in the initial stage, then the reference value of the outer loop output circuit is limited to I H_ref . Therefore, the constant current charging is realized. As the voltage rises, the output value of the voltage outer loop is less than I H_ref . At this time, the output current reference value of the outer loop is input to the current inner loop to realize constant voltage charging. The effect is shown in Fig 11. The working state of the CLLC resonant converter is more complicated, so the extended description function method is used to simplify the nonlinear state equation of its model, and linearization is performed. Finally, the transfer function of the CLLC resonant converter switching frequency to output voltage is obtained as follows [19]: Here, R 0 is the load resistance, and the expression of K vf is as follows: The Bode plot of the system transfer function without the PI controller is shown in Fig 12. The gain of the system at the low-frequency end is low, and the cut-off frequency is too small.
Since the system is a Type 0 system before being corrected, it needs to be compensated to increase the system level so that the system drops at a slope of -20 dB/dec and crosses the 0 line, thereby increasing the phase angle margin of the system and improving the stability of the system. The PI controller transfer function expression is as follows: After adding the PI controller, its closed-loop transfer function expression is as follows: Carry on the double closed-loop PI parameter tuning to Eq (29), obtain the voltage outer loop, and the current inner loop controller expression as follows: The Bode plot of the system transfer function after PI correction is shown in Fig 13. The crossover frequency is 675 rad/s and the phase margin is 64.5˚. This system has a relatively stable mid-frequency band width and has strong anti-interference ability against high-frequency signals.

System simulation
According to the system parameters and control strategy, a simulation model of a dual-output port on-board charging system is built, as shown in Fig 14.   Fig 15 shows the PFC input voltage and current waveforms. It can be seen from the figure that after adopting the dual closed-loop control strategy based on the average current method in the simulation, the input current of the circuit is a sine wave in the same phase with the input voltage (single-phase 220V AC). Fig 16 shows the power factor value, it can be seen that the power factor is greater than 0.99. Fig 17 shows the PFC output voltage waveform. It can be seen from the figure that the output DC bus voltage can be stabilized at about 400V and its ripple coefficient is less than 2.5%. Fig 18 is a diagram of the harmonic content of the PFC input current. From the figure, it can be seen that the THD of the input current is 3.35%, the content of harmonics above the third is relatively low, and the suppression effect on harmonic interference is obvious. Fig 19 shows the operating waveforms of the primary-side power switches Q 1~Q4 when the system operates in parked charging mode. Before the drive signal arrives, the drain-source voltage of the power switches has been pulled to zero potential by the body diode to realize ZVS and reduce the switching loss. Fig 20 shows the waveforms of resonant current I L , excitation current I m , and the center point voltage U ab of the primary side at full load. When the output voltage is 400V, the switching frequency is basically equal to the resonant frequency (100kHz). At this time, the magnetizing inductor does not participate in resonance and presents a triangular wave shape, while the primary-side resonant capacitor and inductor both participate in resonance. Furthermore, the resonant current I L is approximately a sine wave.

Simulation waveform of high-voltage charging circuit
Fig 21 shows the output voltage waveform when the output voltage is 400V and the equivalent load is 50 O. It can be seen from the figure that the output voltage is stable at about 400V at the time of steady state. Set a 5% input voltage fluctuation at 0.6s and the output voltage returns to stability after a short period of oscillation. It can be seen that the fluctuation range is less than 2%, and it has a strong anti-interference ability.  It can be seen from the figure that during the charging process, the state of charge (SOC) of the battery rises steadily and the stable charging current is about 11A. When the bus voltage is low, the battery first undergoes a short discharge and then starts charging. The charging voltage finally stabilizes at 370V. input voltage fluctuation in 0.6s and the output voltage will return to stability after a slight oscillation. Furthermore, the voltage ripple will be less than 0.1%.

Experimental verification
Based on the simulation model, an experimental prototype of a dual-output port on-board charging system was built. The experimental setup is illustrated in (Fig 25). When the driving voltage U gs arrives, the drain-source voltage U ds of the switch has been pulled to zero potential by its body diode, realizing the ZVS of the power switch of the original side circuit.
Figs 29 and 30 show the output voltage waveform of the high-voltage side and low-voltage side of the converter under full load condition. It can be seen that under the double closed-loop PI control strategy, the voltage rise is relatively stable and reaches the peak value within 0.5s. Fig 31 shows the component loss comparison diagram of the converter operating at two operating frequencies under full load condition. It can be seen that the total loss of the converter operating at 0.5 times resonant frequency (50kHz) is 114.8W, and the working efficiency is 96.52%. At the resonant frequency point (100kHZ), the total loss of the converter is 163.2W, and the working efficiency is 95.05%, which meets the design requirements.  It can be seen that the charging efficiency of the high-voltage side circuit under half-load condition is 95.37%, the full-load efficiency is 95.11%, and the maximum working efficiency is 97.86%. Under the condition of half-load, the charging efficiency of the low-voltage side circuit is 94.96%, the full load efficiency is 95.03%, and the maximum working efficiency is 95.76%, which meets the design requirements.

Conclusion
This paper designs a dual-output port on-board charging system based on the CLLC resonant converter. The front stage of the system uses dual-diode bridgeless PFC and the latter stage uses dual-output port CLLC resonant conversion that integrates high and low voltage charging circuits on a transformer. Based on the fundamental wave analysis method, an equivalent model of the CLLC resonant converter is established, the gain characteristics are analyzed, the key parameters of the system are calculated, and the system control strategy is analyzed. Experimental results show that the on-board charging system can achieve power factor correction, has a high power factor, can charge high-voltage power battery packs and low-voltage batteries in the full voltage range, and can achieve soft switching in both modes.